Optical fibers have been used as a medium for transmitting light over long distances in applications such as communications. More recently near infra red (IR) optical fibers have been used in gas sensing applications to pipe light to inaccessible or hostile locations. This has allowed multiple point remote gas sensing techniques to be employed. Such laser based gas sensing techniques typically use a single mode laser source to provide high sensitivity and high-resolution measurements. To preserve the spectral properties of the source, the laser output must be transmitted through single mode fiber.
Recently, quantum cascade lasers have been used in gas sensing applications, as described in WO03087787, the contents of which are incorporated herein by reference. The increased cross sections associated with spectroscopic transitions in the mid IR can provide significantly enhanced detection sensitivities. Fiber based mid IR sensors, however, have not typically been used in gas sensing applications due to the excessive losses associated with single mode mid IR fibers. Many of these losses arise because single mode fibers are very narrow, for example, having core diameters around 5-10 μm, which makes it difficult to efficiently couple light into the fiber. In contrast, multimode fibers typically have core diameters of 400-500 μm. However, transmission of single mode laser light through a multimode fiber results in multiple spatial modes being supported. Such spatial modes typically interfere at the detector, producing optical interference noise.